2017 2nd International Conference on Environmental Science and Engineering (ESE 2017) ISBN: 978-1-60595-474-5

Responses of Inorganic Nitrogen of Forest Soils to in Pearl River Delta, South

Juan HUANG1,2, Jiang-ming MO1,2,*, Xi’an CAI1,2, Wei ZHANG1,2 and Tao ZHANG3 1Guangdong Provincial Key Laboratory of Applied Botany, Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems, Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, China 2State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China 3Institute of Tropical Pratacultural Science, Zhanjiang Normal University, Zhanjiang 524048, China *Corresponding author: Jiangming Mo E-mail: [email protected] Address: No.723 Xingke Road, Tianhe District, Guangzhou, 510650, China

Keywords: Urbanization, Nitrogen deposition, Nitrate enrichment, Pearl River Delta region.

Abstract. Urbanization results in high N deposition and impacts N cycling of forest ecosystems, which increases soil inorganic N pool, including soil N availability, especially for nitrate. However, it is still unknown it is true in south China where ammonium always dominates in soils for long-term traditional agricultural activities. We investigated the changes in soil inorganic N availability responding to urbanization in Pearl River Delta (PRD), China, one of the largest urban areas of the . Soil total inorganic N levels did not show an obvious growth in urbanized (urban and urban/suburban) sites compared with non-urbanized (suburban/rural and rural) sites based on non-significant differences in their concentrations, but nitrate enriched. Our study demonstrated that urbanization resulting in extra N input does not significantly enhance soil total inorganic N availability instead of the changes in its composition, indicating that N cycling in this region is accelerated and its N status is N-rich even N-saturated, consequently potential ecological risks might happen, including nitrate leaching, eutrophication, and plant preference for nitrate. Therein N pollution dominated by nitrate should be paid special attention in China.

Introduction Urbanization characterized by high densities of population and industry is a big feature of today’s world development, especially in developing countries, e.g., and Africa. As a result, new with most densely population are located in the tropics [1]. For example, China has experienced a dramatic and unprecedented increase in urbanization since 1978 for the economic reform, and the urban population rose from 18% in 1978 to 45% in 2008, and up to 54.77% in 2015 [2]. And the Pearl River Delta (PRD) region as one of economic development center of China has overtaken Tokyo as the world’s largest urban area for its urban population reached 42 million compared with the population in Tokyo in 2015 [3].

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Urbanization producing important sources for reactive N compounds, in particular from meat consumption, sewage from huge urban residents and from fossil fuel combustion in automobile engines [4, 5], and greatly increases atmospheric N deposition levels [6, 7]. Higher N deposition with nitrate dominance was observed in urban than in rural areas [7, 8]. N deposition affects soil inorganic N pool via directly inputting ammonium or nitrate or indirectly impacting global biogeochemical N cycles [9]. N deposition entering the forest ecosystems, ammonium can be retained on cation exchange sites in soil organic matter and clays, while nitrate is more mobile and can be uptaken by plants and leaching or gaseous losses during nitrification [10]. Obviously, soil inorganic N availability is not immune to urbanization. The research showed that urbanization increases soil inorganic N availability [11, 12], especial for nitrate, which always dominated in urban soils [11] and exhibits an increase pattern from rural to urban core [13]. In addition, soil inorganic N pool is also affected by environmental factors, such as, temperature, humidity, precipitation, soil pH, etc. Temperature positively affects soil inorganic availability [14]. High temperature helps ammonium to volatile to become ammonia entering the atmosphere, and accelerates the process of N-cycling, including the rates of mineralization, nitrification and loss as gaseous or leaching [15]. The research in the subtropical forest ecosystem showed that nitrification accounted for close to 80% of the net mineralization, and where nitrate was the main N source [16]. Precipitation enhances the loss of soil inorganic N via leaching [17, 18]. Soil pH is closely related to nitrate leaching because of its contribution to soil acidification [19]. Therefore, high N deposition in tropical region with high temperature, heavy precipitation and low soil pH will accelerate the process of N cycling and favor N leaching. Soil nitrate keeping a significant increase is considered as a signal of N-saturated ecosystems as N leaching [20]. Enriched nitrate occurs in urban areas suggested potential N loss and N saturation [12, 20]. N saturation is a condition where N input to the ecosystems exceeds plant and microbial demand [21]. High N deposition reduces biotic demand of subtropical forest ecosystems [22] and changes their N status from N-limited ecosystems into N-rich or N-saturated ecosystems. N saturation was identified in the subtropical forests of south China [6, 23], showing nitrate dominance in soils. Therefore, soil inorganic N pool can be used to understand the state of N + saturation and evaluate the susceptibility of forests to N inputs [15]. Ammonium (NH4 ) - and nitrate (NO3 ) are two main soil inorganic N forms absorbed by plants easily. Soil inorganic N dominated by ammonium or nitrate has a big impact on plant preference for ammonium or nitrate [24]. Hence, the availability and composition of soil inorganic N control the composition of plant species [25], the primary production, biodiversity, structure and function of forest ecosystems [26, 27]. However, it remains unknown whether urbanization enhances soil inorganic N availability and enriches nitrate in China, especial for the Pearl River Delta (PRD) region located in the tropics. China is a country with traditional farming practice [28], which makes ammonium as the dominant N form in soils [29]. But, our recent studies in the Pearl River Delta (PRD) region of China showed that higher N deposition was observed in urbanized areas than in non-urbanized areas and the composition of N deposition changed from ammonium dominance into nitrate dominance in urbanized areas [7]. N-saturated ecosystems were also identified in this region [6, 23]. Moreover, this region has subtropical monsoon climate with high temperature and abundant precipitation, and acidic lateritic earth at regional scale, which helps N loss via leaching or nitrous oxide emission [14, 18, 30].

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- Under condition of high soil moisture, NO3 can be converted by anaerobic denitrifying bacteria to N2 gas or nitrous oxide (N2O), a greenhouse gas. Therein, the responses of soil inorganic N availability to urbanization should be very sensitive in the region. Here, the PRD region, south China was chosen to study the changes of soil inorganic N pool responding to urbanization. We hypothesized that: (1) urbanization enriches nitrate in urbanized sites and changes the composition of soil inorganic N with nitrate dominance; (2) urbanization does not significantly increase soil total inorganic N availability for potential N loss via leaching, indicating the N status of ecosystems in urbanized sites is N-rich even N-saturated.

Materials and Methods

Study Sites and Experimental Design The study area is located throughout Province, south China. In this region, it has a warm and humid climate with annual precipitation ranging from 1566 to 2133 mm and mean annual air temperature from 19.65 to 22.22 °C. Obvious environment gradients caused by urbanization occurred in this region, including: (1) higher air temperature in the core of PRD than in its surroundings [31, 32], (2) higher annual average precipitation in urban areas than in its soundings [33], (3) higher N deposition [7] and lower soil pH [34] in urbanized areas than in rural areas. Four urbanized gradients, including urban, urban/suburban, suburban/rural and rural, expanding across the range of 260 km from the core of Guangzhou to the remote areas along a southeast direction according to the main wind were designed in this region. The detailed description of this design can see [35] and [7, 34]. Fourteen sites were chosen for the study. Among them, three sites were in the urban sites (Huolushan, Maofengshan and Shunfengshan, abbreviated to HLS, MFS and SFS, respectively), four in both the urban/suburban sites (Heshan - HS, Dinghushan - DHS, Guangyinshan - GYS, and Xiangtoushan - XTS) and the suburban/rural sites (Heishiding - HSD, Shimentai - SMT, Yunjishan - YJS, and Dachouding, DCD), and three in the rural sites (Huaiji - HJ, Dadongshan - DDS, and Wuzhishan - WZS). The longitudes of these sites ranged from E111°54′19.78″ to E115′21′54.52″, and their latitudes from N22°46′0.60″ to N24°46′40.25″. Two forest types including a pine plantation (PF) and a neighbored evergreen broadleaved forest (BF) in each site were studied. PF have a wide distribution in South China, accounting for 45% of total plantation area in Guangdong Province [36], and is very vulnerable and sensitive to environmental changes [37]. BF is the typical forest types in south China [38]. The forests with native species Schima superba as one of dominative tree species were chosen. The stand ages were between 40 and 60 years for PF and between 50 and 70 years for BF, and their stand density were all between 600 and 800 trees ha-1. So the two forest types have experienced urbanization since 1978 and exhibited typical responses of the forests in this region. The forest soils were lateritic red earth [34, 35]. Soil Sampling and Measurement Soils sampling was conducted from January to May in 2011, and the samplers from the two forests in each site were collected at the same day. Seasonality was less significant driver of urban soil N levels compare with human distance (i.e. cover type and distance to the urban center) [13]. Thus, the samples of soil inorganic N were only collected

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once. In each forests, three random subplots (5 m×5 m) were selected to sample soil from three mineral soil layers (0-10 cm, 10-20 cm and 20-40 cm) using a 10 cm inside diameter corer. Soil samples were immediately passed through a 2-mm sieve to remove roots and stones, mixed thoroughly by hand after being transported to the laboratory. Fresh soil samples were extracted by KCl solution and filtered to measure the contents + of inorganic nitrogen. The NH4 -N concentrations were measured by the indophenol - blue method followed by colorimetry, and NO3 -N concentrations were directly measured by spectrophotometer [39]. Total inorganic N contents were the sum of the concentrations of ammonium and nitrate. + - Data of atmospheric inorganic N deposition (including NH4 -N and NO3 -N deposition) and soil pH in PF derived from [7] and [34], respectively. Data of stage age, mean annual temperature and mean annual precipitation of each site were collected from the records of the bureau meteorology. Elevation, longitude and latitude of each site were recorded by a GPS device. Statistical Analysis Linear regressions of soil inorganic nitrogen contents to distance from the urban core were determined, and one-way analysis of variance (ANOVA) was used to compare the differences in soil inorganic nitrogen contents among four urbanization classes (urban, urban/suburban, suburban/rural, and rural) to generalize the availability of soil inorganic nitrogen responding to an urbanized environment. The contents of soil inorganic N were analyzed by three-way ANOVA’S with forest types, urbanization gradients and soil depth as factors. Pearson correlation analysis was also performed to examine the relationships between soil inorganic nitrogen with soil pH, mean annual temperature (MAT), mean annual precipitation (MAP) and N deposition. All analyses were conducted using SPSS 13.0 for windows, with statistical significant difference set with P value <0.05, unless otherwise stated. Mean values are expressed ± 1 standard error of the mean.

Results

Variations in Soil Total Inorganic N Soil total inorganic N was in the range of 10.8-65.2 mg kg-1 in PF and of 12.2-63.8 mg kg-1 in BF, there were non-significant differences between in PF and in BF. And there were no significant differences in soil total inorganic N among four urbanization gradients (e.g. urban, urban/suburban, suburban/rural and rural sites) in both BF and PF, except at 0-10 cm in PF, where the mean value of soil total inorganic N was significantly higher in rural sites (P = 0.04) than urban, urban/suburban and suburban/rural sites (Fig. 1). Significant effects of soil depth on soil total inorganic N were observed both in PF and in BF with the highest levels at 0-10 cm and the lowest at 20-40 cm depth. Patterns of Soil Ammonium and Nitrate Levels along Urbanization Gradients In PF, ammonium reduced proximity to urban core and showed a significant positive linear relationship (P < 0.01 for both 0-10 cm and 10-20 cm depth, P < 0.05 for 20-40 cm depth), while nitrate had not clear linear from urban core to rural (P > 0.05) (Fig. 2 and Table 1). Ammonium levels were significant lower in urban and urban/suburban areas than in rural areas (at 0-10 cm depth) or in suburban/rural areas (at 20-40 cm depth) (Table 2). Nitrate levels accounted for 68%, 65% and 63% of the total inorganic

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N in urban sites, and for 55%, 54% and 55% of the total inorganic N in urban/suburban sites at 0-10 cm, 10-20 cm and 20-40 cm depth, respectively, and were below 34% of total inorganic N in suburban/rural and rural sites, but had no significant differences among the four urbanization gradients. Urban 70 A Urban/suburban Suburban/rural 60 a Rural 50 a

) a -1 40 a a a 30 a a a a a 20 a

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Total inorganic nitrogen kg inorganic nitrogen contents (mg Total a 30 a a a 20 a

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Figure 1. Variations in the averages of soil total inorganic N in BF (A) and in PF (B). Error bars indicate (N=3 for urban and rural, N=4 for urban/suburban and suburban/rural). Different letters indicate significant differences (P ) between different urbanization gradients, and same letters indicate no significant differences (P ) between different urbanization gradients, respectively.

0-10 cm 50 a 10-20 cm 20-40 cm 40 Linear Fit of 0-10 cm

) Linear Fit of 10-20 cm -1 Linear Fit of 20-40 cm 30

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0 50 b

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0 250 200 150 100 50 0 Distance (km) Figure 2. Soil inorganic nitrogen contents (N=3) in pine forests along the distance from the urban core of Guangzhou city, China. a), ammonium nitrogen; b), nitrate nitrogen. Ammonium in BF indicated a similar trend to that in PF with a decline near to the urban core, and nitrate exhibited an opposite pattern to ammonium, showing an increase proximity to urban core. The significant relationships between ammonium and nitrate with proximity to urban core were observed at both 0-10 cm and 10-20 cm depth (P < 0.01 for ammonium, P < 0.05 for nitrate), but not at 20-40 cm depth (P > 0.05) (Fig. 3, Table 1). The composition of inorganic N in BF was also similar to that in PF, where the average levels of nitrate were higher than ammonium in both urban and urban/suburban sites, but lower than ammonium in both suburban/rural and rural sites at 40-cm soil depth. Significant differences in the average ammonium concentrations were observed between urban and urban/suburban sites with rural sites at both 0-10 cm and 10-20 cm depth (P < 0.05), but not at 20-40 cm depth (P > 0.05); while significant difference in the average nitrate N concentration was only found between urban sites with suburban/rural and rural sites (P < 0.05) at 0-10 cm depth (Table 2).

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Table 1. Linear regression of soil inorganic nitrogen contents in PF to the distance from the urban core of Guangzhou city, China. Forest type Soil inorganic nitrogen Soil depth Slope Intercept R P value Pine foerst Ammonium nitrogen 0-10 cm 0.156 -0.053 0.787 0.000 10-20 cm 0.106 -0.160 0.673 0.008 20-40 cm 0.107 0.059 0.607 0.021 Nitrate nitrogen 0-10 cm 0.054 17.660 -0.053 0.857 10-20 cm 0.027 12.070 -0.160 0.584 20-40 cm 0.028 8.790 0.059 0.840 Broadleaved forest Ammonium nitrogen 0-10 cm 0.168 -3.149 0.704 0.005 10-20 cm 0.103 -0.504 0.745 0.002 20-40 cm 0.044 5.030 0.349 0.221 Nitrate nitrogen 0-10 cm -0.100 27.205 -0.644 0.013 10-20 cm -0.054 17.490 -0.549 0.042 20-40 cm -0.047 15.400 -0.425 0.130

0-10 cm 60 a 10-20 cm 20-40 cm 50 Linear Fit of 0-10 cm

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0 250 200 150 100 50 0 Distance (km) Figure 3. Soil inorganic nitrogen contents (N=3) in broad-leaved forests along the distance from the urban core of Guangzhou city, China. a), ammonium nitrogen; b), nitrate nitrogen.

Table 2. Mean concentrations of soil inorganic N in PF and BF among four urbanized areas Forest Inorganic N Soil depth Urban areas Urban/suburban Suburban/rural Rural areas Types species (cm) areas areas PF Ammonium 0-10 6.84±1.27b 10.33±3.47b 23.40±6.95ab 32.75±7.79a 10-20 5.53±1.16a 8.73±2.31a 20.71±8.22a 20.44±3.25a 20-40 4.67±1.05b 6.76±2.13b 24.80±8.73a 17.69±1.78ab Nitrate 0-10 18.23±6.64a 16.67±6.71a 10.22±3.92a 22.23±12.59a 10-20 18.23±2.70a 16.67±4.41a 10.22±1.30a 22.23±5.75a 20-40 8.51±2.24a 8.97±3.32a 11.98±5.31a 8.12±3.05a BF Ammonium 0-10 8.04±2.20b 8.10±2.49b 26.66±9.23ab 34.32±10.64a 10-20 5.16±1.73b 7.93±2.63b 17.11±4.37ab 22.59±6.383a 20-40 4.70±1.82a 12.27±7.32a 12.02±2.37a 13.59±4.34a Nitrate 0-10 25.41±9.35a 16.22±4.79ab 7.40±1.10b 8.53±2.13b 10-20 14.90±4.25a 13.02±4.57a 7.50±2.42a 5.99±0.82a 20-40 9.97±2.51a 15.32±6.07a 5.52±1.36a 5.32±1.12a Error bars indicate (N=3 for urban and rural, N=4 for urban/suburban and suburban/rural). Different letters indicate significant differences (P ) between different urbanization gradients, and same letters indicate no significant differences (P ) between four urbanization gradients, respectively.

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Effects of Soil Depth, Forest Types and Urbanization Gradients on Soil Inorganic N Soil depth had a significant impact on the levels of ammonium and nitrate (P < 0.05). Urbanized gradients also showed a significant effect on ammonium N (P < 0.01) and a marginally significant effect on nitrate N (P = -0.07). Forest types did not have a significant impact on soil inorganic N concentrations. And there were not interactive effects on both ammonium and nitrate from soil depth × forest types, urbanized gradients × forest types and soil depth × urbanized gradients × forest types (Table 3). The results suggested that soil depth and urbanization instead of forest type greatly impacted soil inorganic N availability in the PRD region. Correlations between Environment Factors and Soil Inorganic N Inorganic N deposition had significantly negative correlations with soil ammonium N levels in both BF (R = -0.715, P < 0.01) and PF (R = -0.792, P < 0.01), and had a significantly positive correlation with nitrate N levels in BF (R = 0.601, P < 0.05), but not in PF (R = 0.176, P > 0.05), showing N deposition had important impacts to soil inorganic N availability. Significantly negative correlations were also observed between soil ammonium N and AMT both in BF (R = -0.557, P < 0.05) and in PF (R = -0.641, P < 0.05), and significantly positive correlation between soil nitrate N and AMT was only in BF (R = 0.556, P < 0.05), suggesting temperature plays an important role in soil inorganic N levels (Table 4). Table 3. Significance of multivariate for soil inorganic nitrogen among soil depth, forest types and urbanized gradients. Source of variance Ammonium N Nitrate N Intercept 0.000 0.000 Soil depth 0.043 0.022 Urbanized gradients 0.000 0.070 Forest types P > 0.05 P > 0.05 Soil depth × Urbanized gradients P > 0.05 P > 0.05 Soil depth × Forest types P > 0.05 P > 0.05 Urbanized gradients × Forest types P > 0.05 P > 0.05 Soil depth × Urbanized gradients × Forest types P > 0.05 P > 0.05 Table 4. Pearson correlation coefficients of soil inorganic N with soil pH at 0-10 cm depth, atmospheric inorganic N deposition, annual mean precipitation (AMP) and annual mean temperature (AMT). Atmospheric N deposition AMP AMT BF Ammonium N -0.715** -0.457 -0.557* Nitrate N 0.601* 0.000 0.556* PF Ammonium N -0.792** -0.326 -0.641* Nitrate N 0.176 0.345 0.000 ** Correlation is significant at the 0.01 level (2-tailed). * Correlation is significant at the 0.05 level (2-tailed).

Discussion

Distance Indicating the Trend of Soil Ammonium and Nitrate from Urban to Rural Soil ammonium and nitrate in BF and ammonium in PF all showed significant linear relationships with the distances near to the urban core (Fig. 3, 4) which supported that

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distance had correlations with soil chemical properties [40]. In this region, distance also had linear relationships with soil pH [34] and organic carbon [35] and atmospheric N deposition [7]. However, soil nitrate N in PF did not indicate significant linear relationship with the distance near to the urban core (Fig. 2); an important reason was that nitrate concentrations correlate with cations in soil solution [41]. In this region, non-significant variations of soil base cations (e.g. Ca2+, Mg2+) along urban-rural gradient [34] was observed in PF. Pine forest under chronic N deposition keeping great loss of nitrate [42] in urbanized areas was another reason. Soil depth greatly impacted on the levels of soil inorganic N, but didn’t change their pattern from urban to rural. The levels of both ammonium and nitrate were much higher values at the topsoil (0-10 cm) than at depth (Fig. 1, Table2), which was consistent with the results from [43]. But there were non-interactive effects from soil depth with forest types and/or urbanization (Table 3). Soil inorganic N levels including ammonium and nitrate in both BF and PF exhibited similar responses to the urbanization gradient (Fig. 2, 3, Table 2). Forest types had non-significant impacts on soil inorganic N levels and non-interactive effects with soil depth and/or urbanized gradients on soil inorganic N levels (Table 3). Therein the characters in soil inorganic N availability of the two forest types were considered as common responses of forest ecosystems to urbanization in the PRD region.

Nitrate Enriching in Urbanized Areas Nitrate was always dominated in soil inorganic N in urbanized (urban and urban/suburban) sites (Table 2), similar results were reported that nitrate was elevated nearer to the urban core [13] and was dominant N form in urban areas [11, 12]. Urbanization producing various urban sources of nitrate and resulting in nitrate enrichment of atmospheric N deposition (> 30 kg N ha-1yr-1, Huang et al., 2015a) contributed greatly to soil dominated nitrate. There was a positive relationship between the concentrations of soil nitrate with atmospheric N deposition (Table 4), indicating N deposition greatly impacted soil nitrate contents [6, 13, 44]. Low soil pH [34]and abundant precipitation also accelerated nitrate enrichment (Table 4) [17, 18] in this region. China has a strong history of traditional , which results in ammonium always dominated in soils [29]. However, it was not the case in this study (Table 2). Going back to our previous researches in this region, the proportion of soil inorganic has been changing from ammonium dominant [45] to ammonium proportion lower than or equal to nitrate in DHS [46], and then to the proportion of nitrate relative to total inorganic N at 0-10 cm depth be higher in urban and suburban sites than in rural sites across a range of 150 km near to Guangzhou [6] in years to come. Combing with the results in this study (Table 2), a change in soil inorganic N was uncovered that nitrate enriches and becomes the dominant N form instead of ammonium in urbanized areas. Extra N input is the real driver for the change of dominant N form in soils. Consequently, N status of forest ecosystems also changes from N-limited into N-rich even N saturation. N saturation in forest ecosystems also helped nitrate dominance [9, [9, 47], e.g. DHS [23]. Therefore, soil nitrate enrichment and dominance might be an important feature of the forest ecosystems responding to . Compared with nitrate concentrations, lower ammonium levels were observed in urbanized areas (Table2), which could be explained that forests receiving high input of + - N deposition could elevate nitrification and convert NH4 into NO3 at low soil pH [19,

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21]. Moreover, higher temperature in urbanized areas decreased soil ammonium levels, according to a negative correlation between AMT and soil ammonium (Table 4) [48] because higher temperature increased ammonium release via volatilisation and fixation via nitrification [49]. The concentrations of ammonia in PM2.5 were very higher [50], which suggested that ammonia is the main fate for ammonium in this region. However, precipitation did not significantly affect soil ammonium levels based on non-significant correlations between AMP and soil ammonium levels (Table 4), which was consistent with [48].

Urbanization not Significantly Enhancing Soil Total Inorganic N Availability Urbanization resulted in extra N input (e.g. N deposition) [6, 7] into soil, and enhanced soil inorganic N levels [13]. However, significant increase in soil total inorganic levels was not observed in the PRD region based on non-significant differences in soil total inorganic N levels among four urbanization gradients except for the soils in PF at 0-10 cm depth with higher level in rural sites (Fig. 1, 2). Similar results were reported in this region, e.g. in HS, where N addition did not enhance soil inorganic N availability [51]. Fang et al. [6] also reported that in broadleaved forests, there were no significant changes in soil total inorganic N availability in urban and suburban sites compared with rural sites. No positive response of soil total inorganic N to urbanization in this region suggested that N status of the forest ecosystems especially in urbanized areas is N-rich, even N-saturated, and excess N could lose via volatized ammonia into the atmosphere [50], nitrate leaching [6] or N2O emission [23, 52], which accelerates the process of N cycle. Some negative effects such as soil acidification, fine root biomass decline [34] also occurred in this region. This result further suggested that N pollution in the PRD region is very serious, and its potential ecological risks might happen. In this region, soil nitrate leaching inevitably happened especially under high N deposition (> 30 kg N ha-1yr-1) [7] and low soil pH (< 4.5) [34] conditions [53]. A strong positive relationship between atmospheric inorganic N deposition and N leaching was reported in BF [6]. High soil nitrate levels (Table 3) also indicated N leaching [54] based on their positive correlation [53]. The conversion of ammonium into nitrate via nitrification also contributed to N leaching [6]. Heavy precipitation [55] and low phosphorus availability [56] could accelerate nitrate leaching in this region. Leached nitrate entering rivers or lakes could contribute to eutrophication [57]. Consequently, soil C storage [13, 35] and plant growth such as fine root biomass declined [34]. Soil nitrate dominance in urbanized sites would change the preference of plants for N, and force plants favoring nitrate instead of ammonia [24]. Moreover, higher CO2 concentration in urbanized sites compared with non-urbanized sites helped plants prefer nitrate N over ammonium N [58]. So the encroachment of nitrate preference for plants and the change in plant community of forest ecosystems might happen in this region.

Conclusions Rapid urbanization in the PRD region didn’t significantly enhance the availability of soil total inorganic N based on no statistical differences in soil total inorganic N levels along urbanization gradients, but changed the composition of soil inorganic N with nitrate dominance in the urbanized areas. Distance from the urban core indicated the trend of soil nitrate and nitrate from urban to rural. And high temperature and rainy weather in this region contributes the spatial pattern of soil inorganic N. Our study

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indicated that the N status of the forest ecosystems in the PRD region is N-rich even N-saturated, and accompanying negative effects including nitrate leaching, eutrophication and the encroachment of plants preferring nitrate, etc. might happen. Therefore, abating N pollution especially for nitrate will be an important target in China in the future.

Acknowledgements This work was financially supported by National Natural Science Foundation of China (Grant No. 41473112 and 41573073), Natural Science Foundation of Guangdong Province (Grant No. 2016A030313154) and Open Fund of State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (No. OGL-201409). We would like to thank Dr. Hao Chen for help in the field work.

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